1. Field of the Invention
The present invention relates to angular velocity sensors. More particularly, this invention pertains to a batch processed silicon beam array angular velocity sensor based upon the Coriolis sensing method.
2. Description of the Prior Art
Numerous arrangements exist for measuring rotation rate about a preselected axis in inertial space. Such apparatus, commonly designated a gyroscope, forms an essential element of any inertial navigation system. Gyroscopes include, for example, complex and difficult-to-manufacture gimballed spinning rotors, strapdown sensors such the ring laser and fiber optic gyroscopes. All of the above-named rate sensing devices are characterized by high cost, large size and power consumption, complexity of manufacture, expense of maintenance.
Other systems exist for measuring an input rotation rate about a preselected axis which are based upon the principle of the Focoult pendulum that was developed over one hundred years ago. A rate sensor based upon that principle, marketed under the trademark "GYROTRON", was developed by the Sperry Gyroscope Corporation. The device, which, as all gyroscopes of the balanced resonant sensor or tuning fork type, provides significantly greater mechanical and operational simplicity than the above-mentioned types, operates on the principle that, when a tuning fork is rotated about its central axis, it possesses an angular momentum that is equal to the product of the rotation rate and the rotational moment of inertia. The reference motion of the tines of the tuning fork changes the moment of inertia cyclically. As a result, the rotation rate must change cyclically in a complementary fashion to conserve the angular momentum. Thus, the physical operation of the tuning fork type sensor is similar to that of a spinning ice skater who spins faster by pulling his arms in and slows down by extending them. Consequently, in a tuning fork sensor the outward-and inward radial vibration of the tines is converted into a rotational vibration whose amplitude is proportional to the input rate. A closed loop vibrating rotation rate sensor is disclosed in U.S. Pat. No. 5,056,366 of Samuel N. Fersht et al. entitled "Piezoelectric Vibratory Rate Sensor."
Many useful applications exist for angular velocity sensors that do not require gyroscopic accuracy. Such applications are found in flight control systems, automobile skid control systems, video camera stabilization and virtual reality systems.
A type of angular velocity sensing device of lesser complexity whose operation also is based upon the measurement of Coriolis forces is disclosed in U.S. Pat. No. 3,520,195 of Stephen W. Tehon titled "Solid State Angular Velocity Sensing Device" and discussed by William D. Gates in an article titled "Vibrating Angular Rate Sensor May Threaten the Gyroscope", Electronics, pp. 130-134 (Jun. 10, 1968). This device comprises a square metallic rod that is suspended at its nodal supports from a metal frame. The rod is driven at its fundamental frequency by a piezoelectric drive electrode fixed to one surface while a piezoelectric transducer that serves as a output signal pickoff is fixed to an orthogonal surface. While simple in design and concept, this device is not suitable for batch processing. Further, the piezoelectric elements that serve, inter alia, as drive and pickoff electrodes are bonded to the metallic surfaces of the vibrating rod by organic adhesive. Such adhesive materials absorb energy, causing a reduction in the Q of the vibrating rod. As a consequence, more energy must be input into the rod for the purpose of overcoming damping forces that work against vibration driving forces.
A variation of the VYRO is disclosed in an article by Brian Dance, "Piezoelectric Ceramic Elements for Compact Gyroscope", Design News, pgs. 113, 114 (Sep. 20, 1993). The device described comprises an angular velocity sensor of Murata Ltd. of the United Kingdom whose sensing element is formed of "ELINVAR", a nickel-chromium steel alloy. The bar may be of either circular or equilateral triangle cross section. A number of piezoelectric elements are fixed to the sensing element. Again, this device is not suitable for batch processing and is subject to performance degradation due to the presence of organic material for bonding the piezoelectric transducer elements to the vibrating sensor.